This invention relates to a vacuum interrupter, a vacuum circuit breaker or a vacuum circuit interrupter, and, more particularly, to a contact material for a vacuum interrupter having improved wear resistance characteristic and large current interruption characteristic.
Contacts for a vacuum interrupter for carrying out large current interruption or rated current make and break in a high vacuum utilizing an arc diffusion property in a vacuum, are constituted of two opposing contacts, i.e., stationary and movable contacts.
Principal characteristics required for such contacts for the vacuum interrupter are as follows:
(1) the welding resistance is good during the current interruption or make-and-break process; PA1 (2) the current interruption characteristic-is good; and PA1 (3) the voltage withstanding capability is good. PA1 (1) the presence of WC facilitates electron emission; PA1 (2) the evaporation of the contact forming material is accelerated by heating of the surface of electrodes due to collision of field emission electrons; and PA1 (3) the contacts exhibit a low chopping current characteristic which is excellent, e.g., for remaining an arc by decomposing a carbide of the contact material by the arc and forming a charged particle. PA1 (1) the content of the highly conductive component (the total amount of Ag and Cu) is from 25 to 70 vol %; PA1 (2) the contact of the arc-proof component is from to 75 vol % wherein said component is at least one carbide of an element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W; and PA1 (3) the arc-proof component having an average grain size of 0.3 to 3 micrometers is present at an average grain distance of from 0.1 to 1 micrometer. In a preferred embodiment of the present invention, an auxiliary component selected from Fe, Co and Ni can be present in an amount of no more than 10 vol %.
These have been regarded as most fundamental three requirements in the prior art. Versatile studies such as studies of new alloy systems, studies of electrode structures and studies of mechanisms have been carried out and such fundamental three requirements have been dramatically improved. Important requirements other than these three requirements are low and stable temperature rise, low and stable contact resistance, good wear resistance and low and stable chopping current values. However, these requirements contradict each other and therefore it is impossible to meet all of the requirements by a single metal. Accordingly, in many alloy materials which have been practically used, at least two elements which compensate mutually inadequate performance thereof have been used in combination to develop alloy materials which are suitable for specific uses at a large current, at a high voltage or at other conditions. Alloy materials having excellent characteristics have been developed. However, demand for a contact material for a vacuum interrupter which withstands/higher voltage and larger current have increased, and a contact material for the vacuum interrupter which entirely meets such requirements has not been obtained.
On the other hand, in recent years, the diversification of load proceeds with increasing the severity of use conditions of demanders. As a result, it is necessary to use a vacuum interrupter which maintains the fundamental three requirements described above at constant levels and further which has excellent characteristics (demand in load of applied circuits and apparatuses). Generally, an interrupter having a better rank is selected from series of vacuum interrupters having standard specifications. This results in the use of a large system and the loss of economy. For example, in such a case, the fundamental three requirements described above must be insured and the large current interruption characteristic must be compatible with wear resistance.
In general, the surface of contacts is remarkably impaired when large current interruption is carried out. This leads to the wear of the material. The contact having such a worn surface leads to many secondary disadvantages during the make-and-break process or during the interruption process. Therefore, it is required that the wear (contact erosion) be small whenever a large current is interrupted, i.e., it is required that the large current interruption characteristic be compatible with wear resistance.
A known contact material which meets the fundamental three requirements is a Cu--Bi alloy containing no more than 5% by weight (hereinafter referred to as wt %) of an anti-welding component such as Bi (Japanese Patent Publication No. 12131/1966). This Cu--Bi contact segregates Bi in crystal boundaries and this therefore renders the alloy per se brittle. Thus, a low welding opening force is realized and the alloys have an excellent large current interruption property.
Japanese Patent Publication No. 23751/1969 discloses the use of a Cu--Te alloy as a contact material which is used for a large current. While this alloy alleviates the problems associated with the Cu--Bi alloy, it is more sensitive to an atmosphere as compared with the Cu--Bi alloy. Accordingly, the Cu--Te alloy lacks the stability of contact resistance or the like. Furthermore, although both the contacts formed from the Cu--Te alloy and those from the Cu--Bi alloy have excellent anti-welding properties in common and can be used sufficiently in prior art moderate voltage fields in respect to voltage withstanding capability, it has turned out that they are not necessarily satisfactory in applying to higher voltage fields.
On the other hand, a known contact material for a vacuum interrupter is a Cu--Cr alloy containing Cr. This alloy contact exhibits preferred thermal characteristics of Cr and Cu at a high temperature and therefore it has excellent characteristics with respect to high voltage withstanding capability and large current interruption characteristic. That is, the Cu--Cr alloy is widely used as a contact wherein high voltage withstanding characteristic is compatible with large capacity interruption characteristic. However, the Cu--Cr alloy exhibits greatly inferior welding resistance characteristics as compared to the Cu--Bi contact containing no more than about 5% of Bi which has been generally utilized as a contact material for an interrupter. Accordingly, operation mechanism by which a vacuum interrupter formed by using a contact of a Cu--Cr alloy is driven requires a larger opening force as compared with the vacuum interrupter formed by using the Cu--Bi alloy contact, and therefore the vacuum interrupter formed by using the Cu--Cr alloy contact is disadvantageous in with respect to miniaturization and economy.
A Cu--Cr--Bi alloy obtained by adding an anti-welding metal such as Bi or Te to a Cu--Cr alloy is known. The welding resistance of the material is remarkably improved by this alloy. However, the amount of Bi evaporated can vary depending upon conditions used during heat treatments such as baking and brazing, and therefore scattering can occur in respects of large current interruption characteristics and wear resistance. When the current of an inductive circuit such as a motor load is interrupted by means of the conventional vacuum interrupter which does not pay due regard to a surge during the make and break process, an excessive abnormal surge voltage is generated and the load instrument tends to break.
The reasons why such an abnormal surge voltage is generated are attributable to phenomena such as the chopping phenomenon generated when a small current is interrupted in a vacuum (a current interruption is forcedly carried out before the waveform of an alternating current reaches the natural zero point) and a high-frequency arc-extinguishing phenomenon.
The value Vs of the abnormal surge voltage due to the chopping phenomenon is expressed by a product of the surge impedance Zo of a circuit and the current chopping value Ic, i.e., Vs=Zo.Ic. Accordingly, in order to reduce the abnormal surge voltage Vs, the current chopping value Ic must be decreased.
In order to meet the requirements described above, there have been developed vacuum switches wherein contacts composed of tungsten carbide (WC)-silver (Ag) alloys are used (Japanese Patent Application No. 68447/1967 and U.S. Pat. No. 3,683,138). Such vacuum switches have been put to practical use.
The contacts composed of such Ag--WC alloys have the following advantages:
Another contact material exhibiting a low chopping current characteristic is an Ag--Cu--WC alloy wherein the ratio of Ag to Cu is approximately 7:3 (Japanese Patent Application No. 39851/1982). In this alloy, the ratio of Ag to Cu which has not been used in the prior art is selected and therefore it is said that stable chopping current characteristic is obtained.
Furthermore, Japanese Patent Application No. 216648/1985 suggests that the grain size of an arc-proofing material (e.g., the grain size of WC) of from 0.2 to 1 micrometer is effective for improving the low chopping current characteristic.
Further, Japanese Patent Laid-Open Publication No. 35174/1978 discloses a Cu--WC--Bi--W alloy wherein the welding resistance of the sintered alloy described above is highly improved.
It is important that the contact material for a vacuum interrupter meets the fundamental three requirements described above and further requirements emphasized by demanders (wear resistance). However, these requirements contradict each other and therefore it is impossible to meet all of the requirements by a single metal material. Accordingly, in many contact materials which have been practically used, at least two elements which compensate mutually inadequate performance thereof have been used in combination to develop contact materials which are suitable for specific uses at a large current, at a high voltage or at other conditions. Contact materials having excellent characteristics have been developed. However, demands for a contact material for a vacuum interrupter which has high reliability have increased, and a contact material for the vacuum interrupter which entirely meets such requirements has not been obtained.
That is, a highly boiling component is advantageous for providing arc-proof property which is relevant with wear resistance. However, the high boiling component exhibits high temperatures when it is exposed to an arc. Accordingly, thermal electron emission is remarkable. Thus, the highly boiling component is disadvantageous and large current interruption cannot be maintained and improved.
In the Cu--Bi contact material described above, the brittleness of a stock is utilized to insure welding resistance. Accordingly, the Cu--Bi contact material has a fatal drawback in respect of wear resistance, surface roughening occurs during the current interruption or make-and-break process and thus the contact resistance characteristic exhibits large scattering.
In the prior art conventional Ag--WC contact material, Ag is selectively evaporated in a relatively early period as the number of current interruption or make and break increases. Thus, portions containing no Ag are locally generated to lead to the increase of contact wear. That is, in the prior art contact composed of WC and Ag, the large current interruption characteristic can be improved by adjusting the amount of WC. However, the amount Ag can relatively vary and therefore wear resistance characteristic changes. Accordingly, it is necessary to make various improvements in order to obtain lower and stable both characteristics even at the same amount of Ag.
In the contacts composed of the WC--Ag alloys (Japanese Patent Application No. 68447/1967 and U.S. Pat. No. 3,683,138), the large current interruption characteristic per se is insufficient, and no regard is paid to the improvement of wear resistance characteristic.
In the Ag--Cu--WC alloys wherein the weight ratio of Ag to Cu is approximately 7:3 (Japanese Patent Application No. 39851/1982) and the alloys wherein the grain size of the arc-proofing material is from 0.2 to 1 micrometer (Japanese Patent Application No. 216648/1985), their wear resistance characteristic is not entirely satisfactory.
In the Cu--WC--Bi--W contact materials, the welding resistance of the Cu--W contacts is improved by a synergistic effect of the presence WC and particularly Bi. However, the scattering of wear resistance characteristic is still observed.
An object of the present invention is to provide a contact material for a vacuum interrupter which combines excellent large current interruption characteristic and wear resistance characteristic and which meets the requirement for the vacuum interrupter to be used under severe conditions.